1. Field of the Invention
The present invention relates to a synchronous detection method and device, which are used for detecting signal components modulated on a target carrier wave contained in an input signal.
2. Description of the Related Art
When separating desired signal components from an input signal with a low signal to noise ratio (S/N ratio) or extracting a target carrier wave with a desired frequency from an input signal containing a plurality of carrier waves having different frequencies with each other, various types of synchronous detection devices have been used. One of which is disclosed in Japanese Unexamined Patent Publication H6-269060, and another one of which is disclosed in U.S. Patent Application Publication No. 2003/0039325A1 corresponding to Japanese Unexamined Patent Publication 2003-65768.
An example of conventional synchronous detection devices is illustrated in FIGS. 19A and 19B. The conventional synchronous detection device is used for reducing at least one carrier wave and a direct current (DC) component, which are components to be eliminated, from an input signal Vsa including a target carrier wave. In particular, the synchronous detection device has an analog synchronous detection circuit 50.
In the synchronous detection circuit 50, the input signal Vsa is separated into two input signals Vsa. One of the separated input signals Vsa is fed to a voltage follower composed of an operational amplifier OP1, which is served as a buffer circuit. The one of the separated input signals Vsa is converted by the voltage follower into a first output signal whose signal level is obtained by multiplying the signal level of the one of the separated input signals Vsa by the gain of “+1”.
The other of the separated input signals Vsa is fed to an inverting amplifier composed of an operational amplifier OP2 and resistors R1 and R2 (see FIG. 19B). The inverting amplifier multiplies the signal level of the other of the separated input signals Vsa by the gain of “−(R2/R1)” to convert it into a second output signal.
Assuming that the resistances R1 and R2 are set to be equal to each other in FIG. 19B, the signal level of the second output signal is obtained by multiplying the signal level of the other of the separated input signals Vsa by the gain of “−1”.
Switches SW1 and SW2 are connected to the output paths of the operational amplifiers OP1 and OP2, respectively. The switches SW1 and SW2 are alternatively turned on in synchronization with a clock signal CK with the same frequency (clock pulse frequency) as the target carrier wave.
The alternative turning on of the switches SW1 and SW2 allows the first and second output signals to be alternatively outputted as an output signal Vo1. That is, the input signal Vsa is alternatively inverted and non-inverted in the half periods (half pulse periods) of the clock signal CK to be outputted as the output signal Vo1.
As described above, when the input signal Vsa includes the at least one carrier wave except for the target carrier wave and the DC component, which are to be eliminated, in addition to the target carrier wave of detection, the conventional synchronous detection device uses the analog synchronous detection circuit 50. The analog synchronous detection circuit 50 eliminates the at least one carrier wave and the DC component. That is, the analog synchronous detection circuit 50 shown in FIG. 19B rectifies both positive and negative half-waves of the input signal Vsa in synchronization with the clock signal CK to convert the negative half wave of the input signal Vsa into a positive half wave, thereby canceling the at least one carrier wave and the DC component.
The output signal Vo1 outputted from the synchronous detection circuit 50 is obtained by rectifying both positive and negative half-waves of the input signal Vsa including the target carrier wave, This results in that high frequency noise components containing fluctuations of the target carrier wave itself and/or noises that are not eliminated by the synchronous detection circuit 50 remain in the output voltage Vo1.
For eliminating the high frequency noise components from the output signal Vo1, as shown in FIG. 19B, the conventional synchronous detection device is provide at the output side of the synchronous detection circuit 50 with an analog low pass filter 52. That is, the analog low pass filter 52 eliminates the high frequency noise components from the output signal Vo1, thereby outputting an output signal Vo2 while stabilizing it.
The conventional synchronous detection device uses the analog synchronous detection circuit 50 having analog amplifying circuits, such as the operational amplifiers OP1 and OP2, to rectify both positive and negative half-waves of the input signal Vsa. This may result in that noise components generated by the operations of the analog amplifying circuits (the operational amplifiers OP1 and OP2) are superimposed on the output signal Vo1 from the synchronous detection circuit 50. The noise components may not be sufficiently eliminated by the low pass filter 52 so that the noise components may remain in the output signal Vo2, which may contribute to the difficulty of the signal detection, in other words the synchronous detection, with high precision.
When implementing the synchronous detection device in a chip, 2 different analog amplifying circuits, such as the operational amplifiers OP1 and OP2, must be implemented in the chip, which may make it difficult to reduce the synchronous detection device in size and in cost. Especially, for maintaining the signal level of each noise component outputted from each analog amplifying circuit within a predetermined target level, a footprint of each analog amplifying circuit on the chip needs to increase. The increase of the footprint of each analog amplifying circuit may also contribute to the difficulty of the reduction of the synchronous detection device in size and in cost.
An analog amplifying circuit, such as an operational amplifier, has characteristics that widely fluctuate depending on the fluctuations of the circuit's part characteristics, such as the characteristics of transistors, the resistances of resistors, and the capacitances of capacitors. These circuit's part characteristics of the analog amplifying circuit are susceptible to environmental changes around the circuit, such as temperature changes. Using such analog amplifying circuits to the synchronous detection device may make it difficult to improve the environmental resistance of the synchronous detection device and to ensure the reliability thereof.
In addition, the analog low pass filter 52 is composed of an operational amplifier, resistors, capacitors, and other components, and especially, the resistors and the capacitors are external components. The characteristics of the operational amplifier, the resistors, and the capacitors of the analog low pass filter 52 are also susceptible to environmental changes around the circuit, which may cause the reliability of the synchronous detection device to deteriorate.